1. Field of the Invention
The present invention generally relates to a method and apparatus to generate a bias voltage and more particularly to a method and apparatus to generate a bias voltage between 3.3V and 12V from a small input voltage of about 1V.
2. Description of the Related Art
With the increasing popularity of integrated circuits that need a bias voltage, there is a demand for the generation of a bias voltage between 3.3V and 12V from a small input Voltage of around 1V which is the operation voltage or core voltage for the core voltage of modern computer processors.
FIG. 1 exemplarily illustrates boost circuit 100 and FIGS. 2 and 3 exemplarily illustrate boost circuit 100 in continuous conduction mode 200 and discontinuous conduction mode 300, respectively. Boost circuit 100 is configured to provide an output voltage Vout that is a bias voltage of about 3.3V to 12V. Boost circuit 100 includes oscillator 110, control 120, and switch 130.
In an on-state, the switch 130 is closed, resulting in an increase in an inductor current I ind through the inductor L. In an off-state, the switch 130 is open and the only path offered to the inductor current I ind is through the diode D. The controller 120 receives feedback from the output of the diode D18 and can control the gate 130 accordingly.
FIG. 2 illustrates a continuous mode 200. When boost circuit 100 operates in the continuous mode 200, the inductor current I ind never falls to zero. Referring to FIG. 3, a problem arises in that, under low load conditions a ringing occurs at the inductor, when the circuit operates in a discontinuous conduction mode 300. As the load current gets smaller, the control circuit 120 lowers the duty cycle and the boost circuit 100 transits into the discontinuous conduction mode 300. Then, current through the inductor I ind reaches zero and at this condition large high frequency oscillations occur. These oscillations can disturb other circuits and also unnecessarily consume energy. But because this circuit is used for low current bias voltages, it will always work with low load currents and is always in the DCCM.
Some integrated circuits meet some of these requirements, but are not ideal. These integrated circuits have the following problems. These integrated circuits have a high cost. In some cases, the costs of these integrated circuits are between three to four dollars. In addition, these integrated circuits are relatively too large, since these independent circuits are designed for load currents >300 mA.
Thus, a major drawback for these integrated circuits is the generation of high frequency oscillations when going into a discontinuous conduction mode.